A stacked gate structure of a memory cell of a NAND flash memory includes a first gate insulating film, a charge storage layer, a second gate insulating film, and a control gate electrode, which are formed in order on a silicon substrate.
In recent years, as the size of a memory cell decreases, an insulating material (high-k film) having a high dielectric constant is under consideration for use as the second gate insulating film.
One of candidates of high-k films for use as the second gate insulating film includes lanthanum oxide and lanthanum oxynitride. Lanthanum oxide and lanthanum oxynitride have the property of easily absorbing water and carbon dioxide. There is a problem in that, when the second gate insulating film using lanthanum oxide or lanthanum oxynitride absorbs moisture and carbon dioxide, the insulating performance is degraded.